An ‘‘amplitude model’’ of the laser-rf double-resonance process has been quantitatively verified, providing a complete explanation of the anomalous line shapes observed in studies of ${\mathrm{Na}}_2$ hyperfine structure. We find excellent agreement with new laser-rf double-resonance measurements in low-J’’ rotational levels of the $X^1$${\Sigma }_g^{+}$(v’’=0) ground state of ${\mathrm{Na}}_2$ as well as with earlier data for J’’=28. These results confirm the idea that absorption and stimulated-emission Rabi cycles in the presence of the laser field create coherence between the ground-state hyperfine levels when these levels are optically unresolved. A novel ground-state ‘‘quantum beat’’ experiment is proposed.

• Received 9 December 1985

DOI:https://doi.org/10.1103/PhysRevA.33.3166